The description in the previous section does fully describe the reasons for using local-as. This BGP feature facilitates the process of changing the ASN of all the routers in a network from one number to another. This may be necessary if one network operator merges with or acquires another network operator and the two BGP networks must be consolidated into one autonomous system.
For example, suppose the operator of the ASN 64500 network merges with the operator of the ASN 64501 network and the new merged entity decides to renumber ASN 64501 routers as ASN 64500 routers, so that the entire network can be managed as one autonomous system. The migration can be carried out using the following sequence of steps.
Change the global AS of the route reflectors that used to be part of ASN 64501 to the new value 64500.
Change the global AS of the RR clients that used to be part of ASN 64501 to the new value 64500.
Configure local-as 64501 private no-prepend-global-as on every EBGP session of each RR client migrated in step 2.
This migration procedure has several advantages. First, customers, settlement-free peers and transit providers of the previous ASN 64501 network still perceive that they are peering with ASN 64501 and can delay switching to ASN 64500 until the time is convenient for them. Second, the AS path lengths of the routes exchanged with the EBGP peers are unchanged from before so that best path selections are preserved.